1. Field of the Invention
The present invention relates to a control device for a stepping motor, and, more particularly to a technique for lead angle control.
2. Description of the Related Art
In general, a control device for a stepping motor uses a pulse signal as a command signal. In other words, the control device commands a position with a pulse number and commands speed with a pulse frequency. Examples of techniques in the past for setting a lead angle value corresponding to the torque of the stepping motor in such a control device are disclosed in Japanese Patent No. 3715276 and Japanese Patent Application Laid-Open No. 11-113289. In a technique 1 disclosed in Japanese Patent No. 3715276 and a technique 2 disclosed in Japanese Patent Application Laid-Open No. 11-113289, a current phase θi of each phase winding is set as described below.
Technique 1
When δθ<90°θi=θcom 
When δθ>90°θi=θfb+90°+KV·ω+Ke·ω·δθ
where θcom is a position command, θfb is a rotor position, δθ is a positional deviation, θi is a current phase, KV is a proportional constant, Ke is a proportional constant, and ω is rotor speed.
Technique 2
When δθ<90°+KV·ωfb θi=θcom 
When δθ>90°θi=θfb+90°+KV·ωfb 
where ωfb is rotor speed.
According to the techniques in the past, the rotor position command and the rotor position are compared. When the positional deviation is within a predetermined range, an excitation phase is set with the position command as a stable point. When the positional deviation exceeds the predetermined range, a lead angle value is set to an optimum value. The optimum lead angle value in the techniques in the past means a lead angle value with which maximum torque can be generated with respect to the speed ωfb.
However, in the techniques in the past, when the positional deviation exceeds the predetermined range, a motor is accelerated by the maximum torque that can be generated. Therefore, rotational speed of the rotor rotating to a position of the position command (a target position) may substantially exceed speed of a speed command (target speed) to cause a deficiency described below.
For example, after a rotor shaft is rotated by an external force, when the rotor shaft is opened, the rotor rotates to return to an original position with the maximum torque. In returning to the original position, it is likely that the rotational speed of the rotor reaches an abnormal speed. Since the rotor is not decelerated until the rotor reaches near the original position, because of the inertia of the rotor and a load, the rotor may pass over a position where the rotor should stop. When the inertia is large, the rotor may be unable to stop while repeatedly passing over the position and moving back. A similar phenomenon can occur because of insufficient acceleration torque during operation, fluctuation in a load, and the like.